Drug delivery is a field of research focused on administrating a pharmaceutical compound to achieve a therapeutic effect in a patient. The biological target, such as a receptor or cell, must be exposed to the drug for a length of time and at a concentration that is adequate to achieve the therapeutic effect. The drug must be administered to the patient, with the common routes of administration being oral, topical, transmucosal, and inhalation routes. There are many different approaches to achieving a controlled release of a drug over time so that the drug may be delivered effectively and conveniently.